The present invention relates to an optical device in which an optical waveguide is formed therein and also relates to a method of producing such an optical device.
An optical waveguide useful for optical communication or the like has heretofore been formed in glass by an ion-exchange process or a flame hydrolysis process.
According to the ion-exchange process, Ag.sup.+ ions are thermally diffused into a surface layer of a glass substrate through a slit -shaped opening of a thin metal film deposited on the glass substrate so as to form an optical waveguide in the surface layer by the first ion-exchange step to replace Na.sup.+ ions in the glass with Ag.sup.+ ions. A uniform electric field is then applied to the glass substrate in a Na.sup.+ fused salt.
Thereafter, the surface layer of the glass is penetrated with Na.sup.+ ions from fused salt. Due to the diffusion of Na.sup.+ ions, a high refractive index region which was formed at the outmost layer by the diffusion of Ag.sup.+ ions is submerged beneath the surface, so that the optical waveguide is embedded below the surface of the glass so as to ensure a characteristic of low propagation loss.
The core of the optical waveguide obtained in this manner generally has a semicircular or circular section of 10-200 .mu.m in radius and a relative index difference of around 1%.
Since the change of refractive index is controlled by ion-exchange, the structure of the formed optical waveguide is limited to the part near the surface of the glass. In addition, the glass materials available for the waveguide formation are limited to those capable of ion-exchange. One further shortcoming is that the ion-exchange requires a relatively long time process resulting in poor productivity.
According to the flame hydrolysis process, minute glass particles are deposited on a silicon substrate by the flame hydrolysis of silicon tetrachloride and germanium tetrachloride, so as to form a duplicated glass layer for an under clad and a core. The accumulated minute glass particles are then reformed to a transparent glass layer by heat treatment at a high temperature. Thereafter, a core part is shaped to a predetermined pattern by photolithography or reactive etching and then a clad is formed.
The flame hydrolysis process involves complicated steps for the formation of an optical waveguide and only uses silica-based materials. In addition, it is difficult to form an optical waveguide having a circular section by the flame hydrolysis process, since the core is formed by etching.
Although an optical waveguide can be formed with various two-dimensional patterns by either the ion-exchange process or the flame hydrolysis process, it is nearly impossible to form channels with a three-dimensional pattern. Consequently, an optical waveguide circuit having a complicated structure cannot be obtained by these processes.
An object of the present invention is to provide an optical device in which an optical waveguide is formed with a high degree of freedom.
Another object of the present invention is to form such an optical waveguide in glass by the structural change which is induced by laser beam irradiation.
The inventors recognized the light-induced effect wherein the structure of glass is changed by laser beam irradiation, and have studied the feasibility of this effect for forming an optical waveguide.